A “tightly-coupled cluster” is a system of multiple machines that share common resources and are typically co-located. When machines are sharing resources, they need to be careful not to perform simultaneous conflicting operations. For example, when one machine is altering a resource, the other machines should not be accessing it. Coordination of their operations on the resource can be done by the use of inter-machine locks.
Conflicting operations, which these inter-machine locks prevent, generally involve shared resources which are accessible by all of the machines in a cluster. Access to the shared resources (e.g., SAN-attached disks) is typically slower than access to local memory (e.g., caches). A machine may save a copy of a shared resource in its local memory. However, the local copy may become invalid if the resource in the shared location is modified by other machines. The local copy can be accessed freely while the resource is protected by an inter-machine lock; but if the lock is released, the resource must be validated upon the next lock acquisition. This validation usually comes from re-reading the shared resource, which is a performance-negative operation.